This invention relates to the field of precision axial solenoids, and more particularly to printing solenoids, such as print wire solenoids for use in dot matrix impact-type printers and hammer-type solenoids for use in daisy-wheel printers.
A typical print head for a dot matrix-type of printer may have either seven or nine wires, each operated by an individual print wire solenoid. High speed operation of such printers may require the ability to produce in excess of 600 characters per second with an average of six dots per character. An individual print wire may be required to produce in excess of 1,000 impacts per second, while maintaining a clear and distinct impact pattern.
Each impact dot produced by the wire represents a complete cycle of operation for the print wire solenoid, in which a coil is energized to move an armature from a rest position to a forward or actuated position. The print wire is carried on or operated by the armature and moved into impact with the printing medium. When the solenoid coil is de-energized, the armature is returned to its rest position by means of a spring. Total movement of the armature usually does not exceed 0.040" and more commonly is in the range of 0.200". The return momentum of the armature must be absorbed with minimum rebound so that the unit is capable of high speed operation, without being out of phase with its electronic signal.
In the mass production of such solenoids it is important that they be designed so as to be produced at low cost and yet provide repeatability of design performance from unit to unit. In other words, it is important to provide a design in which the speed of operation and force of application will remain within desired limits for all units manufactured throughout a production run. One critical design parameter of a solenoid of this type is the air gap spacing between the armature and stator. It is important that the working air gap, across which the motive force is generated, be accurately maintained from unit to unit. In the past, threaded external adjustments have been provided through which a desired air gap could be reestablished after the solenoid has been assembled. The problem of maintaining a precise internal air gap has resulted from the difficulty in controlling the stack-up of the tolerances of the many assembled parts, the total axial variations of which result in a loss of control of the desired air gap dimension within the assembled part.
The problem defined above with respect to the maintenance of a precise air gap in printing solenoids is also related to the problem of the maintenance of a fixed rest position for the solenoid armature with respect to a reference datum plane which does not change during the use of the solenoid. The datum plane may be established by a rebound absorbing material in applications where control of rebound or bounce is critical. Generally speaking, the control of rebound or bounce is significant in printing solenoids, but the maintenance of a given datum plane or rebound surface by use of an elastomer has proven in the past to be difficult due in part to the thermal expansion of the elastomeric material, and the fact that the elastomeric material has a tendency to take a set after the solenoid has been in use for a period of time.